The present invention is directed to an automatic choke unit for an internal combustion engine and, more particularly, it is directed to a control arrangement for maintaining the position of the choke valve so that the optimum volume of air is supplied to the venturi in a carburetor depending both on engine temperature and the ambient atmospheric pressure about the engine.
In conventional automatic choke units, a bimetal member for controlling the choke valve is generally located within a casing into which air is passed which has been heated by the exhaust gas from the engine and is drawn into the casing by the negative pressure in the engine intake system. As the bimetal member is heated by the air flowing into the casing, it controls the extent to which the choke valve is opened. However, if a vehicle containing such an automatic choke unit runs up a long hill, or if the vehicle continues to run at high speed or when its engine is running at full or high load, the negative pressure in the intake system drops and, as a result, the warm air passing into the casing decreases in volume and the temperature in the casing drops off. As the temperature in the casing drops, the bimetal member cools off and the choke valve is displaced toward the closed position even though the engine is sufficiently warmed-up. As a result, the air-fuel ratio becomes enriched and engine operation is adversely affected, fuel consumption is increased, and the pollutants or contaminants in the exhaust gas are also increased.
Furthermore, these conventional automatic choke units are arranged to increase the air-fuel ratio before the engine is sufficiently warmed-up so that positive startup and smooth running of the engine is attained. However, when a vehicle is running at a high elevation, the ambient atmospheric pressure drops and the air-fuel ratio tends to become enriched. Moreover, when the engine is running, its output drops so that the engine must run at a higher load than at which it runs under normal atmospheric pressure. Accordingly, the negative pressure in the intake system drops and the amount of warm air flowing into the casing decreases in volume. Under such conditions, the bimetal member cools off even though the engine is sufficiently warmed-up and the choke valve is moved toward the closed position. Further, the conventional choke units cannot control the air-fuel ratio in dependence on the temperature of the engine whereby the engine operation is adversely affected.